1. Technical Field of the Invention
The present invention relates generally to a fault detecting apparatus for a gas concentration sensor which is designed to detect and identify different types of faults of the sensor accurately.
2. Background Art
NOx sensors are known which measure the concentration of nitrogen oxides (NOx) contained in exhaust emissions of automotive engines. Such sensors are typically used in an emission control systems for lean burn internal combustion engines equipped with a NOx catalyst absorbing NOx emitted during lean burn of the engine. The NOx sensor is installed downstream of the NOx catalyst. When the quantity of NOx absorbed in the NOx catalyst increases or sulfur emissions contained in the exhaust gas are adhered to the NOx catalyst, thus resulting in decreased ability to absorb the NOx, the emission control system finds such an event through an output of the NOx sensor and enriches an air-fuel ratio of a mixture supplied to the engine to recondition the NOx catalyst.
Japanese patent First Publication No. 11-14589 (equivalent to EP 0887640) teaches a diagnostic system working to diagnose faults in a gas concentration sensor. This system makes such a diagnosis using the fact that the temperature of the gas concentration sensor bears a correlation to an ac resistance (i.e., impedance) thereof. Specifically, when a heater installed in the sensor is energized so as to bring the ac resistance to within a given reference range, but the ac resistance lies out of the reference range, the diagnostic system determines that any fault has occurred in the sensor.
Japanese Patent First Publication No. 11-37972 (equivalent to EP 0892265) also teaches a diagnostic system for a gas concentration sensor equipped with a main pump cell, a first gas chamber, and a second gas chamber. A feedback control system works to control a pumping operation of the main pump cell to adjust a partial pressure of oxygen within the first gas chamber to a level not to decompose NOx gases. If the feedback control system is controlled using a correcting circuit, but it is difficult to bring the concentration of oxygen within the second gas chamber into agreement with a target level, the diagnostic system determines that the gas concentration sensor is malfunctioning for some reason.
The diagnostic systems, as described above, have the drawback in that it is impossible to locate the fault in the sensor. Specifically, diagnostic systems of this type are capable of detecting a failure in operation of the sensor itself, but cannot take action about the failure. For example, it is possible to take action to correct sensor control schemes without replacing the sensor itself in some types of faults, but however, the above diagnostic systems have a difficulty in identifying such an event.